Re: [Phys-L] The Ingenious Watt Balance (and why we need it)

["Donald Polvani" <dgpolvani@verizon.net>]

> And please let me know if you think there's anything wrong with it too, so
I
> can provide clarifying remarks with the video if necessary!
>
> Derek McKenzie
> http://PhysicsFootnotes.com

First, I found the video interesting and informative.  However:

1) It did move too quickly for me.  Perhaps, this is just a result of my
aging senior citizen mind (I'm a retired PhD physicist who has taught
undergraduate level electricity and magnetism), but I found that I had to
replay the video several times to satisfy myself that I thoroughly
understood it.
2) As one example, when explaining the two "platform" coils, the person's
finger was touching the two opposing magnets mounted on each base (which I,
at first, mistook for the "platform" coils).  This caused confusion.
3) The first part says that the ampere is defined, in part, by the kilogram,
but now, with the watt balance, we can define the kilogram, in part, by the
ampere.  Don't we now have to change the definition of the ampere, so it
doesn't include use of the kilogram?  Explanation of this point would help.
4) There were other sections I found that I had to repeat to fully
understand.  I was especially bothered by the use of the term "Laplace"
force for the magnetic force acting on the "platform" coil.  It bothered me
that the term "Lorentz" force or "magnetic" wasn't used instead and caused
me to feel that I had missed something.  Looking it up in my old
undergraduate textbook (written in the 60's), I found that the term
"Lorentz" force strictly applies to the force produced on moving charges
when both electric and magnetic forces are present. However, the text added
that sometimes the term "Lorentz" force is used for just the magnetic
portion of the force.  This has been my practice.  Checking several modern
textbooks I have used to teach electricity and magnetism, I found that the
term "Lorentz" force is reserved for the entire electric/magnetic force and
the magnetic term is referred to simply as the "magnetic" force.  I've never
seen the magnetic force referred to as the "Laplace" force. This,
admittedly, small point threw me off in understanding what was going on
until I replayed the video several times.  Presumably, students learning the
material for the first time wouldn't have a problem with "Laplace" versus
"Lorentz" versus, "magnetic" force.  Perhaps, electrical engineers use the
term "Laplace" force?  At any rate, simply causing the force opposing the
weight force the "magnetic" force would be helpful, I believe, to old and
young alike.

Again, it was interesting and informative to watch.  Thanks, for submitting
it.

Dr. Donald Polvani
Adjunct Faculty, Physics, Retired
Anne Arundel Community College
Arnold, MD 21012